Ferrous pellets

ABSTRACT

The production of ferrous metallurgical pellets containing sufficient reductant for complete reduction has been accompanied by problems, because the pellets tend to crumble at temperatures in the range of 300*-800* C. This loss of hot crushing strength is suggested herein to be due to the reduction of iron oxide from the trivalent to the divalent state. In accordance with this invention, the reductant is subjected to a controlled oxidation which removes the most active carbon, preventing significant reduction at temperatures below about 900* C. and which preserves the hot crushing strength. Oxidation of the reductant can be carried out before or after pellet formation.

United States Patent Wienert [451 Apr. 4, 1972 1 F ERROUS PELLETS Fritz0. Wlenert, 394 Roosevelt Avenue, Niagara Falls, NY. 14305 [63]Continuation-impart of Ser. No. 464,545, June 16,

1965, abandoned.

[72] Inventor:

[52] U.S. Cl ..75/28, 75/33, 75/44 8/1966 Ban ..75/5 2/1967 Ban ..75/3

Primary ExaminerHyland Bizot Assistant Examiner.l. E. LegruAttorney-Ashlan F. Harlan, Jr.

[57] ABSTRACT The production of ferrous metallurgical pellets containingsufficient reductant for complete reduction has been accompanied byproblems, because the pellets tend to crumble at temperatures in therange of 300800 C. This loss of hot crushing strength is suggestedherein to be due to the reduction of iron oxide from the trivalent tothe divalent state. ln accordance with this invention, the reductant issubjected to a controlled oxidation which removes the most activecarbon, preventing significant reduction at temperatures below about 900C. and which preserves the hot crushing strength. Oxidation of thereductant can be carried out before or after pellet formation.

9 Claims, No Drawings FERROUS PELLETS RELATED APPLICATIONS Thisapplication is a continuation-in-part of U.S. application Ser. No.464,545 filed June 16, 1965 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to novel pellets containing iron oxide and carbon that aresuitable for metallurgical use in the production of metallic iron. It isalso concerned with novel procedures for producing such pellets.

It has long been desired to produce metallic iron from fine iron oxideor iron ores. Hitherto this has been accomplished by forming pellets offine iron oxide and reducing the iron oxide to metallic iron by heatingthe pellets to temperatures higher than about 900 C. in an atmosphere ofreducing gases. The reduction is slow since the reducing gases mustdiffuse into the pellets and the process is rather expensive because ofthe high temperature involved, the slowness of the reaction, and thecost of the reducing gases.

2. Prior Art It has previously been suggested to incorporate finelydivided carbon or carbonaceous materials in pellets formed from ironores. Such suggestions have not, however, been fruitful since whenheated to temperatures above 300 C. the pellets become so weak and softthat they break up into dust when handled. This has precluded anypractical application of such pellets in the production of metalliciron. Other attempts have been made to utilize pellets containing bothiron oxide and carbon or carbonaceous material, but prior to the presentinvention none has been practical. Some of these attempts are discussedbelow.

Dahl et al. (U.S. Pat. No. 3,323,901) form pellets of ore, carbon, waterand a binder which is either sulfite lye or molasses. The pellets aredried with combustion gases in two stages, first at a temperature up toabout 90 C. and then at a temperature up to about 150 C. The pellets areprereduced in a kiln at 900-l000 C. To facilitate the escape of moistureduring drying, the patentees prefer relatively large ore particles.

The U.S. Pat. Nos. of Ban (3,264,091 3,304,168) disclose conventionalpelletizing of ore, carbon and fluxes followed by drying with inertgases at 1503l6 C. (300-600 F.), preheating at 538l,093 C. (l,0002,000F.) and firing to produce highly metallized pellets at 1,316 C. (2,400F.). These operations-are carried out on a travelling grate withfrequent reversals of gas flow direction so that the entire bed istreated. Ban would prefer a completely neutral gas (N, CO but realizesthat oxygen-depleted air (preferably less than 10 percent is thecheapest, he economically makes use of it. Further, he recognises thatthe air will decompose some of the coal and pick up CO and hydrocarbons,which must be burned to keep the gas from becoming reducing. He assumesthat the treatment will result in a 20 percent fixed carbon loss andadds reductant with this in mind.

It is, accordingly, an object of the present invention to producepellets consisting essentially of particulate iron oxide and carbon witha carbon content high enough to reduce at least a major portion of theiron oxide when the pellets are heated to temperatures above about 900C. in a suitable metallurgical furnace.

Another object of the invention is to provide pellets of the characterdescribed which produce metallic iron on heating without the need forthe diffusion of reducing gases into said pellets.

A further object of the invention is to produce pellets of the characterdescribed which retain sufficient strength during heating as to permittheir effective and practical utilization in a suitable metallurgicalfurnace for reduction of the iron oxide to metallic iron.

Other objects and advantages of the present invention will be apparentfrom the following description thereof.

The foregoing objects are accomplished by forming the pellets in suchmanner that reaction between the iron oxide and the carbon therein isretarded, particularly in the temperature range from about 300 C. toabout 800 C. Such pellets mayibe heated without disintegration insuitable metallurgical furnaces to temperatures well above 1,000 C., atwhich temperatures the reduction of the iron oxide to metallic ironproceeds at a rapid rate.

As used herein, unless otherwise specified, the term pellets refers tobodies compacted by pressure, bodies formed by the so-called ballingaction from particulate material and a liquid, and also to bodies formedfrom fine materials by other suitable methods. Although with pelletsproduced according to the present invention the size of pellets is notlimited by difficulty of obtaining proper diffusion of a reducing gasinto the pellets, with excessively large pellets the transfer of heat tothe core thereof may. be very slow. Accordingly, it is generallypreferred to produce pellets having diameters in the range from 10 mm.to 25mm.

As will be apparent from the foregoing, iron oxide-carbon pellets madein the usual manner, though strong enough to stand, without difficulty,handling at room temperatures and temperatures up to about 300 C.,become so weak at higher temperatures that they crumble into powderunder extremely light pressures. On the other hand, if iron oxide-carbonpellets do not disintegrate until a temperature of about 800 C. isreached, recrystallization, crystal growth, and/or partial fusionapparently become effective to strengthen the pellets and permit them tobe used without difficulty in metallurgical furnaces.

It has been discovered by the present inventor that the structure ofprior iron oxide-carbon pellets is weakened in the intermediatetemperature range from about 300 C. to 800 C. by the reduction oftrivalent iron oxide in the pellets to divalent iron oxide.

One of the most effective ways of obtaining iron oxide-carbon pelletswith good strength when heated to 800 C. was found to comprise theemployment as a carbonaceous reductant of a material which has arelatively low reactivity at temperatures up to about 800 C. This isillustrated in the following example in which the particles ofmetallurgical coke used as a carbon source are subjected to preliminary,controlled, limited oxidation at temperatures up to 800 C. in air havinga normal oxygen content to decrease their activity in this temperaturerange. It has been found that the control of the oxidation shouldpreferably be such as to result in a loss of fixed carbon by combustionof between 1 percent and 10 percent. The reason for reduced reactivityof the reductant after this treatment is believed to be that the carbonoxidized (and lost) is the most active carbon available, i.e. carbonwhich would readily form CO. Further, the treatment also drives offremaining volatile matter, reducing available H As is known, H and COcan reduce iron oxide from the trivalent to the divalent state attemperatures above about 300 C., whereas reduction by solid carbon doesnot proceed at an appreciable rate until about l,000 C. is reached.Additionally, increasing the ash to carbon ratio makes the carbon lessreactive because the ash remains around the remaining carbon. In each ofthe examples below, the oxidation was carried out in air having anordinary oxygen content so that no inert or neutral gases need be usedfor reducing or are even desirable therefor.

EXAMPLE 1 Metallurgical coke was ground to pass a 24 mesh screen and theportion passing a 200 mesh screen was discarded. The 24 mesh +200 meshcoke particles were treated by heating them in a current of air toapproximately 800 C., the weight ratio of air to coke being 1.63110. Thetreated coke was then cooled in nitrogenas an inert atmosphere.

Magnetite ore, with an analysis of 67.2% Fe, 3.8% SiO 0.5% MgO, and 0.6%A1 0 was ground to the following particle size distribution:

A batch consisting of 100 parts of the ground magnetite, 22.4 parts ofthe treated coke, and 4 parts of limestone (24 mesh) was pelletized intoballs about 20 mm. in diameter with the help of an aqueous solution ofsodium silicate. After airdrying, sample pellets were found to have acrushing strength of about 15 lbs. The air-dried pellets were placed ina container having a lid with a venting hole. The container was insertedin an electrically heated furnace in which the pellets were heated toabout 800 C. in a period of 50 minutes. Then the pellets were cooled toroom temperature in a protective atmosphere such as nitrogen. The hotcrush strength, i.e. the force required for crushing, of samples ofthese heated pellets was then determined in the manner describedhereinafter, and found to average 11 lbs.

Another effective way, similar to that described above, to produce ironoxide-carbon pellets with good hot crush strength involves controlled,limited oxidation of the particles of carbon after the pellets areformed. This is illustrated below.

EXAMPLE 2 One hundred parts of hematite ore ground to pass a 100 meshscreen was mixed with 22.4 parts of ground metallurgical coke (24 mesh+150 mesh) and 4 parts 24 mesh ground limestone. This mixture waspelletized with the aid of an aqueous sodium silicate solution and theresultant green pellets, while still moist, were coated with a mixturemade from 12 parts of fine hematite (-24 mesh) and 6 parts of 65 meshmetallurgical coke. The coated pellets were air-dried and were thenheated to about 800 C. in 50 minutes while 6 parts of air flowed aroundthe pellets. The average hot crush strength of the heated pellets was 13lbs.

In the procedure of this example the small, controlled amount of oxygenpresent during the heating resulted in retarding of the reduction of theiron oxide at the temperatures involved and thus the strength of thepellets was maintained. Each heated pellet was found to have a hardershell about 6 mm. thick around a softer core about mm. in diameter. Thehardness of the shell decreased from the pellet surface toward thecenter illustrating the effect of diffusion of oxygen into the pellets.

In the following two examples a combination of the procedures of theforegoing examples is employed:

EXAMPLE 3 Anthracite having the composition: 85 percent fixed carbon 5.5percent volatiles, 8 percent ash, 0.5 percent sulfur, l percent moisturewas ground and screen. The 24 mesh +200 mesh portion was placed in anelectric furnace and heated in 30 minutes to about 800 C. in a reducingatmosphere. While holding the anthracite at this temperature for 15minutes 5 percent of air was admitted to produce a controlled, limitedoxidation of the carbon. The heating also reduced the amount ofvolatiles present. The treated material was then cooled quickly innitrogen.

A mixture of 100 parts hematite (200 mesh), 24 parts of the treatedanthracite, and 4 parts of 24 mesh ground limestone was pelletized withan aqueous sodium silicate solution and the resultant pellets wereair-dried. The dried pellets were heated without access of air in anelectric furnace to about 600 C. in 15 min. Then-l7 parts of air wereintroduced during 35 minutes while the temperature was raised to 800 C.

The heated pellets were cooled in a nitrogen atmosphere and I the hotcrush strength determined. An average strength of 14 lbs. was found. Theheated pellets had a relatively hard shell about 4 mm. thick.

mesh +35 mesh 35 mesh +100 mesh 2.01% l00 mesh +l50 mesh 3.6% l50 mesh+200 mesh 68.4% 200 mesh Pellets were made from 15.5 parts of water anda mixture of 100 parts of the hematite ore, 16 parts of the oxidationtreated metallurgical coke described in Example 1, 2.75 parts of 24 meshlimestone and 4 parts of bentonite clay, the latter serving as a binder.

After air-drying, the pellets were heat treated in an electricallyheated furnace by raising the temperature of the furnace to 800 C. in 60minutes.During the first 30 minutes of heating 4 parts of air wereadmitted to the furnace and during the second 30 minute period, 8 partsof air were admitted. After cooling the treated pellets in a nitrogenatmosphere, the hot crush strength of samples was found to average 19lbs.

In producing pellets of iron oxide and carbonaceous material accordingto the present invention, it has been found that obtaining high hotcrush strengths in such pellets is aided and facilitated by employingrelatively large particles of the carbonaceous material. For example,when employing ground metallurgical coke with hematite ore the use ofcoke of 24 mesh +200 mesh size gives pellets with a hot crush strength66 percent greater than pellets in which the coke included a substantialamount of particles smaller than 200 mesh in size. An increase in hotcrush strength of 166 percent was obtained when coke of 24 mesh +65 meshsize was used. That satisfactory, even improved, pellets can be obtainedwhen using relatively large particles of iron oxide and/or carbon wasnot to be expected from prior known practices with ilmenite and chromiteores since it had been thought essential for practical reduction thatthe particles in the pellets be extremely fine.

The use of relatively coarse carbon particles in producing ironoxide-carbon pellets is illustrated below.

EXAMPLE 5 Metallurgical coke was ground to pass a 24 mesh screen and theportion passing a 65 mesh screen was discarded. The remainder was mixedwith hematite ore which had been ground to such fineness that overpercent passed a 200 mesh screen and with limestone ground to pass a 24mesh screen in the proportion of parts ore, 22.4 parts coke, 4 partslimestone. Pellets about 20 mm. in diameter were formed from the mixtureby a known method using an aqueous solution of sodium silicate as abinder. The pelletswere dried, heated in 50 minutes to about 800 C. inthe absence of air, and cooled in a protective atnosphere. The averagehot crush strength of the heated pellets was 8 lbs.

It will be seen that the hot crush strength of the pellets resultingfrom Example 5 is somewhat lower than that of the pellets of Examples14. While for general use in metallurgical furnaces pellets with a hotcrush strength of at least 10 lbs. are preferred, in some cases pelletswith a lower hot crush strength are usable. For example, a hot crushstrength as low as 3 lbs. is satisfactory for pellets that are heatedfor reduction to metallic iron on a traveling grate, although a greaterstrength is usually preferred. Under such conditions the pellets aresubjected to relatively small stresses during reduction.

The carbon content of pellets produced according to the presentinvention may be derived from any convenient source subject to thelimitations set out herein. Thus anthracite, graphite, and metallurgicalor other coke, such as petroleum coke may be used. As stated above, thecarbon should be present in pellets accordingly to the invention'in anamount sufficient to reduce at least a major portion of the iron oxideto metallic iron when the pellets are heated to temperatures above about900 C. More carbon can be used, however. In fact, in some instances itmay be desired to use carbon in an amount exceeding up to 10 percent theamount necessary for reduction of all of the iron oxide in the pellets.

It has been determined that, in general, carbonaceous material such asmetallurgical coke which has a relatively low volatile content will givehigher hot crush strength in pellets forming from comparable mixes thancarbonaceous materials like bituminous coal that have a high volatilecontent. This is understandable, in the light of this invention, sincethe volatiles will either contain reducing gases (H CO, etc.) or producethem by reaction with carbon, and the gases will reduce the oxide to thedivalent state in the 300-800 C. range. This effect is quite noticeable.For example, it was found that pellets formed from a hematite-bituminouscoal mixture without treatment according to the present invention were,after heating to 800 C., too weak to be removed from the containerwithout crumbling. On the other hand, a similar hematite-metallurgicalcoke mixture gave pellets which had a hot crush strength of 5 lbs. Ingeneral, the content of the volatile matter in the carbon source shouldnot exceed about 4 percent if satisfactory hot crush strength is to beobtained. However, as indicated hereinbefore, the particle size of theiron oxide and carbon and the treatment of the carbon during thecontrolled, limited oxidation thereof also influence the strength.

The process of the present invention is applicable not only to hematiteore and magnetite ore as illustrated in the foregoing examples, but isalso usable with pure or relatively pure iron oxides from other sourcesand with other iron ores such as goethite and limonite. The principlesof the invention may also be applied to improve the hot strength ofpellets made of carbon and ilmenite or chromite.

In pelletizing the iron oxide-carbonaceous reductant mixture, anydesired process may be employed. The production of pellets fromparticles is well known and a variety of suitable methods and apparatusare available. Preferred binders for use in pelletizing are sodiumsilicate, bentonite, and other suitable non-reactive materials, althoughother binder materials may be used. It is generally desirable, however,to avoid binders such as tar or pitch which on heating form volatilereducing products.

The hot crush strength of pellets was determined by the followingmethod:

A single pellet was placed on a vertically adjustable metal stem clampedto a spring balance and, by adjusting the stem, was raised into a small,vertical tube furnace through which argon was circulated. The furnacewas heated electrically with resistance wire and the temperature of thepellet was raised to 800 C. in about 3 minutes. A piston was thenlowered into the furnace into contact with the pellet and pressure,readable as weight on the balance, was slowly applied to the pelletthrough the piston. Breakage of the pellet was indicated by a suddendecrease in weight as shown by the spring balance.

Iron oxide-carbon pellets produced according to the present inventionthat have a hot crush strength of at least about lbs. may be heated toproduce metallic iron by reduction at temperatures from about l,000 C.to about 1,250 C. in a rotary kiln, passing therethrough incountercurrent to flame gases. The strength and character of suchpellets permits their use in this way without a special abrasionresistant coating or the employment of finely divided cushioningmaterial. Consequently, it is possible to charge the reduced pelletsdirectly from the rotary kiln into a melting furnace, thus preservingthe sensible heat of the reduced pellets and making additional handlingunnecessary. As pointed out above, with careful handling, pellets havingsomewhat lower hot crush strength are also suitable for reduction undercertain conditions.

As shown in the examples, it is usually desirable to add a small amountof a particulate lime-bearing material such as limestone, dolomite,calcium hydroxide, or the like to the pellet mix to combine with sulfurpresent in the carbon source and/or in the reduction furnace atmosphere.The lime-bearing material may be omitted if desired. When used, it maybe present in such amount as is required to form calcium sulfide withthe sulfur in the pellets and to form calcium silicates with silicapresent in the pellets as an impurity. Further, if desired, a smallamount of fine lime-bearing material of the type ooze from the pelletsat hi h temperature. I

Mesh sizes referred to erern were determined with standard Tylerscreens. Percentages and parts specified are, unless otherwiseindicated, percentages and parts by weight.

It will be apparent that the present invention is susceptible to certainvariations and modifications. It is intended, therefore, that it shallbe interpreted as broadly as permitted by the appended claims.

What is claimed is:

1. A process for producing metallurgical pellets having a hot crushstrength of at least 3 lbs. and consisting essentially of particulateiron oxide and particulate carbon, which are substantially non-reactivewith each other in a reduction reaction at temperatures up to about 800C., and yielding metallic iron upon heating above 900 C. in ametallurgical furnace. the carbon being distributed throughout saidpellets in at least such proportion to the iron oxide as to reduce themajor portion of said iron oxide to metallic iron, characterized in thatthe particles of carbon are subjected to controlled, limited oxidationby heating them, prior to appreciable reduction of the iron oxide insaid pellets, to a temperature of about 800 C. in the presence of alimited amount of oxygen.

2. The process of claim 1, wherein said oxidation is controlled toproduce a loss of fixed carbon by oxidation of between 1 percent and 10percent.

3. A homogenous pellet consisting essentially of:

particulate iron oxide, substantially unreduced; a small amount of abinder;

a particulate carbonaceous reductant having less than 4 percent volatilematter, in an amount great enough to reduce at least a major portion ofsaid iron oxide;

said reductant having had removed therefrom 1 percent to 10 percent ofits fixed carbon content by controlled 0xidation at a temperature in therange from 300 C. to 800 C., and, in said condition, being incapable ofsignificant reduction of said iron oxide at temperatures below about 900C.; and

said pellet having a hot crush strength of at least 3 lbs.

4. A pellet as defined in claim 3, in which the particles ofcarbonaceous reductant are larger in size than 74 microns.

5. A pellet as defined in claim 3, in which a substantial portion of thearticles of iron oxide are larger in size than 74 microns.

6. A pellet as defined in claim 3, which additionally contains a smallamount of a particulate lime-bearing material.

7. In a process for producing hard, substantially unreduced pelletsconsisting essentially of particulate iron oxide an amount ofparticulate carbonaceous reductant sufficient to reduce at least aportion of said iron oxide and having less than 4 percent volatilematter, optional added fluxing material and a small amount of a binder,said process comprising mixing and pelletizing said ingredients, theimprovements comprising heating said carbonaceous reductant to atemperature in the range of from 300 C. to 800 C. in the presence of alimited amount of oxygen to oxidize between 1 percent and 10 percent ofthe fixed carbon of said reductant, whereby said pellets may be heatedto about 900 C. without significant reduction of the iron oxide therein.

8. A process as defined in claim 7, in which the heating of theparticles of carbonaceous reductant is carried out prior to forming saidpellets.

9. A process as defined in claim 7, in which the particles ofcarbonaceous reductant are heated after formation of said pellets.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 3,653,876' V Date d April 4, 1972 Inventor(s) Fritz O Wienert It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Claim 3, line 6, after "oxide", insert to metallic ir0n Claim 7, line 4,after "oxide", insert --'to metallic iron line 5, insert a comma after"material" Signed and sealed this 2nd day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents F ORMPO-105O (10-69) USCOMM-DC 60376-P69 a uvs, GOVERNMENT PRINTING OFFICE:"ls 0-3664 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 53, 8 6 Dated A ril 4, 1972 Inventor-( Fritz O. Wienert It iscertified that error appears in the above-identified patent and thatsaid Letters Patent 'are hereby corrected as shown below:

Claim 7, line 4, by-inserting "major" before ortion;

Signed and sealed this 22nd day of August 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD MGFLETGHERJR.

Commissioner of Patents Attesting Officer FORM PO-iOSO (10-69) uscoMM-oc60376-P69 fi' U.S, GOVERNMENT PRINTING OF ICE: 1969 0-366-334

2. The process of claim 1, wherein said oxidation is controlled toproduce a loss of fixed carbon by oxidation of between 1 percent and 10percent.
 3. A homogenous pellet consisting essentially of: particulateiron oxide, substantially unreduced; a small amount of a binder; aparticulate carbonaceous reductant having less than 4 percent volatilematter, in an amount great enough to reduce at least a major portion ofsaid iron oxide; said reductant having had removed therefrom 1 percentto 10 percent of its fixed carbon content by controlled oxidation at atemperature in the range from 300* C. to 800* C., and, in saidcondition, being incapable of significant reduction of said iron oxideat temperatures below about 900* C.; and said pellet having a hot crushstrength of at least 3 lbs.
 4. A pellet as defined in claim 3, in whichthe particles of carbonaceous reductant are larger in size than 74microns.
 5. A pellet as defined in claim 3, in which a substantialportion of the articles of iron oxide are larger in size than 74microns.
 6. A pellet as defined in claim 3, which additionally containsa small amount of a particulate lime-bearing material.
 7. In a processfor producing hard, substantially unreduced pellets consistingessentially of particulate iron oxide an amount of particulatecarbonaceous reductant sufficient to reduce at least a portion of saidiron oxide and having less than 4 percent volatile matter, optionaladded fluxing material and a small amount of a binder, said processcomprising mixing and pelletizing said ingredients, the improvementscomprising heating said carbonaceous reductant to a temperature in therange of from 300* C. to 800* C. in the presence of a limited amount ofoxygen to oxidize between 1 percent and 10 percent of the fixed carbonof said reductant, whereby said pellets may be heated to about 900* C.without significant reduction of the iron oxide therein.
 8. A process asdefined in claim 7, in which the heating of the particles ofcarbonaceous reductant is carried out prior to forming said pellets. 9.A process as defined in claim 7, in which the particles of carbonaceousreductant are heated after formation of said pellets.